Use mDecorWindowContext to verify window resources

Fixes an issue where two different Contexts are used to create an
AdditionalWindow and verify the SurfaceControl.Transactions on its
SurfaceControl.

The window is created using mDecorWindowContext, which may have an
overriden density and may be different that the density of the
display. Meanwhile, the verification phase was using the Context of
ShellTestCase.java, which is a TestableContext whose density will be
true to the display density. This CL changes that the verification
phase to also use mDecorWindowContext. It also changes the Context
object passed into the constructor of TestWindowDecoration from
Instrumentation.getContext() to the TestableContext for consistency.

Bug: 286259007
Test: `atest WindowDecorationTests`, then from settings change the
display size, then running `atest WindowDecorationTests` again.

Change-Id: I8d61f6836c7088efd5d77edf494066b2a2b12f8a